1. Design Patterns Part 1: Observer, Singleton, & Adapter

2. Learning Objectives Design patterns: what & why When and how to apply
Familiarity with several popular patterns:
Observer
Singleton
Adapter

3. What is a Design Pattern?
Template solutions to common problems
Benefits:
Avoid re-inventing solutions
Promotes software quality
Reusability, maintainability, etc.
Facilitate communication

4. Running Example: Tiger Dining

5. Tiger Dining Web App

7. Design problem:
notification

8. Chef’s view

9. Manager’s view: To see subscribed users

10. How to connect?
Orders
notification

11. OrdersController
new()
create() …
Notifier
order_notification() ?

12. Solution: Observer Pattern
Subject
attach(observer)
detach(observer)
notify()
Observer
update()
observes *
Needs to add an observer for the food order activity
Define one-to-many dependency

13. order_notification() update()
Subject
attach(observer)
detach(observer)
notify()
Observer
update()
observes *
OrdersController
new()
create()
add_observer()
notify_observers()
Notifier
order_notification()
update()
observes *

14. Control Flow OrdersController Order EmailNotifier create save
notify_observers
Left to right
update
Send

15. Goal: Implement Observer pattern
Modify OrdersController (Subject)
include “Observable”
adds observer using “add_observer”
Uses “notify_observers”
Modify Notifier (Observer)
Observer has “update” method
Add code snippet

16. Goal: Implement Observer pattern

17. Goal: Implement Observer pattern

18. Other Examples
Spreadsheet
Employee payroll
GUI program (callback)

19. Classroom Activity

20. Design problem:
Sales Logging

21. Problem Saves records of transactions as backup
Sales logging for transaction failure

22. SalesLogger
initialize(file)
formatter
log(message)

23. Solution: Singleton Single file to log all the sales information
Easy to look up
Singleton
inst: Singleton
instance(): Singleton …
if inst == nil
inst = Singleton.new
end
return inst

24. Making class Singleton
Steps of making a class Singleton:
Creating the class variable
Initializing it with the single instance
= SimpleLogger.new
Creating the class level instance method
Making a new private

25. Goal: Implement Singleton pattern
SalesLogger class
include “Singleton”
SalesController
call instance
call log(message)

26. Goal: Implement Singleton pattern

27. Singleton Examples In Rails: Outside Rails: development.log test.log
Webrick in rake
Outside Rails:
tomcat
ant

28. Design problem:
New Payment method

29. Goal: To Add Visa Debit Card and Paypal

30. Goal: To add VisaPay PaymentsController new create … DiningDollars
charge(uid) …

31. Solution: Adapter Interface which bridge a gap in payment methods
Key objects are Target, Adapter and Adaptee
Target
Request()
Client
Adapter
Request()
Adaptee
SpecificRequest()

32. DiningDollarsAdapter
PaymentProcessor
process_payment()
PaymentsController
DiningDollarsAdapter
process_payment()
VisaPayAdapter
process_payment() …
DiningDollars
charge(uid)
VisaPay
pay(debitcard, cvv, exp) …

33. Visa processes the payment
Control Flow
PaymentsController
VisaPayAdapter
VisaPay
create
process_payment
pay
Visa processes the payment

34. Goal: Implement Adapter Pattern
PaymentsController
“create” method call the adapter based on the input type
VisaPayAdapter
defines “process_payments” inherited from the PaymentProcessor
VisaPay
defines pay(creditcard, cvv, exp) saves the information

35. Adapter: Example in Rails
Active Records
Contains adapters for several databases (PostgreSQL or MySQL)
Wraps them up in common interface
AbstractAdapter implements common functionality
PostgreSQLAdapter
AbstractMysqlAdapter

36. Summary Singleton: Observer: Adapter: Making Sure There Is Only one
Keeping Up with the Times
Adapter:
Filling the gaps

37. Practice Quiz